1. Field of the Invention
The present invention relates generally to a method of, and an apparatus for, automatic quality control in the manufacture of geometrically regular-shaped products, such as tube and rounds, over a wide range of temperatures and, in particular, relates to a method and apparatus that incorporates nondestructive dimensional analysis by means of penetrating radiation and computer models of the product.
2. Description of Prior Art
Technology in many fields has advanced to the level that materials are being pushed to their limits. For this reason, quality control and assurance is becoming increasingly important, not only for production efficiency, but also to prevent failure of a part of a device which could, if undetected, potentially cause loss of property and lives.
In the quality control of any product, it is preferable to test each finite segment of each item of product rather than merely make a random sample. In order to test every product, it is essential that the test be continuous and nondestructive and it is preferable that the test be performed quickly enough to provide real-time control so as not to inhibit the production process.
In the automatic process control of any production process it is also preferable that feedback and feedforward signals for quality control purposes be based on detailed sequential information about every product in the production line, rather than on a random sample of the total product produced. In order to gather data on each unit of product, it is essential that the testing be done so as not to interrupt the production process.
In process control, it is preferable that quality control feedback and feedforward signals be provided as near to real-time as possible. The faster feedback control signals can be delivered to the production process, the less out-of-specification product is produced. In a hot production process this requires that the testing data be collected when the product is at elevated temperatures. The ability to test the product-in-manufacture at elevated temperatures is especially important if control signals are fed forward to additional production processes which can correct the defect while the product remains at elevated temperatures.
In the past, penetrating radiation has been used in several quality control applications. Several of these inventions, as exemplified by U.S. Pat. No. 3,248,916, disclose the use of penetrating X-rays to gauge the thickness of sheet metal. Although U.S. Pat. Nos. 3,841,123 and 3,851,509 teach the use of penetrating X-rays to gauge the final thickness of sheet metal, they both disclose that, even in such simple production processes, the use of X-ray gauges is too slow for automatic process control systems.
One prior art process control system uses X-rays in the production of more complex products. In U.S. Pat. No. 3,496,745, the process control device uses X-rays to measure the average wall thickness of a tube after the last rolling stand and provides feedback. However, this device only provides feedback with respect to one dimensional measurement: the average wall thickness. This testing regimen significantly limits the applicability of the invention to a comprehensive process control system for the production of dense, geometrically-complex products, such as tube or pipe.
U.S. Pat. No. 4,725,963 discloses an apparatus that performs continuous three-dimensional analysis of complex products. However, the patent does not disclose a device that can perform this analysis in real-time on each tube as tubes are produced. Therefore, the system is not capable of being incorporated into a production control process which provides feedback and feedforward information in real-time as the product is being produced.
The invention disclosed in U.S. Pat. No. 4,725,963 is also limited by its inability to consider shrinkage of the product being produced. Although the patent discloses a system that can take measurements of tube at elevated temperatures, it does not correct those measurements to account for shrinkage of the product which accompanies cooling from elevated temperatures.
Further, U.S. Pat. No. 4,725,963 does not include elements necessary for analyzing the measurements generated to identify the types of flaws in the product or their causes and also does not include means for generating, from the dimensional analysis, control signals to modify the manufacturing process for quality control.
U.S. Pat. Nos. 3,496,745 and 4,535,614 disclose systems which suggest adjusting the manufacturing process based on longitudinal flaws. These prior art control systems automatically adjust the roller settings for each product produced to account for the differences between the forces that are imposed on the leading and trailing end of a workpiece. However, the previously disclosed systems assume that the same adjustment is needed for each tube in response to previously developed empirical data. The present invention makes individual adjustments based on the individual data of each workpiece.
In the past, automatic control systems have used means for performing dimensional analysis other than X-rays. U.S. Pat. Nos. 3,841,123 and 3,851,509 illustrate the use of "force gauges on sheet metal." On the other hand, the system in U.S. Pat. No. 4,771,622 uses magnetic detectors on sheet metal. Magnetic detectors are inapplicable to complex shapes, such as tubes or pipes, and are limited to use on materials subject to magnetic detection. Since steel does not possess its magnetic properties at temperatures above the curie temperature, this apparatus cannot be used with a hot steel rolling process. The present invention is applicable to magnetic as well as nonmagnetic materials.
Other systems, as exemplified by U.S. Pat. No. 4,535,614, contemplate the use of a light source and light sensors to measure the shadow of a product. This method is obviously limited to information about the outer surface of an object and is therefore inapplicable to tube and other similarly-shaped products because it cannot detect cavities in such objects.
Historically, multiple process control systems considered the temperature of the workpiece and shrinkage. The invention disclosed in U.S. Pat. Nos. 3,841,123, 3,851,509 and 3,592,031 use temperature data in sheet metal rolling process control systems. However, these systems all made previously determined adjustments to production processes based on the measured temperature. Additionally, all of these systems were simple in that they were concerned with only one dimension of the product: its thickness. The prior systems did not calculate the corrections from the unique metallurgical properties applicable to the specific batch of raw material being used.